Microwave frequency separator



Jan. 15, 1963 P. G. SMITH 3,074,033

MICROWAVE FREQUENCY SEPARATOR Filed Feb. 26, 1957 2 Sheets-Sheet 2 TERM/IVA L 2/ TERMINAL 22 INVENTOR. GENE SM/TH Y HJZM ,4 TTOR/VE V nit This invention relates to a microwave frequency separator, and more particularly to such a device wherein the separated frequency circulates as a traveling wave around a transmission line loop.

In microwave communications systems and in microwave radar systems it is sometimes required that one system be able to simultaneously transmit or receive two or more microwave signals which are at different frequencies. In systems of this type it is desirable that the signals at different frequencies be separated from each other with a minimum of interference from the other signals.

Therefore the need arises for a device which is capable of separating the signals at various frequencies and at the same time providing isolation between the different channels of a multi-frequency system.

There are known devices which are capable of separating microwave signals at different frequencies. Many of these devices employ hybrid junctions, or resonant cavities, or rely on reflection of energy to obtain frequency separation. However, these known devices suffer from one or more of the following limitations: High selectivity cannot be obtained between the different frequencies; the devices are large and complicated to construct; the impedances at the different terminals. of the device are not constant over a band of frequencies.

Therefore, it is an object of this invention to provide a device which is highly selective in separating microwave signals of different frequencies.

It is another object of this invention to provide a microwave frequency separating device which presents a substantially constant impedance match at all terminals over a relatively broad band of frequencies.

It is another object of this invention to provide a microwave frequency separator which is small in size and weight.

These and other objects of the invention which will become apparent as the description proceeds are achieved by a device comprising a waveguide in the form of a closed loop having first and second waveguides coupled to the loop by directional couplers. Signals of two or more microwave frequencies are coupled into one terminal of the first waveguide. The waveguide loop is an integral multiple of a waveguide wavelength long at one microwave frequency and an integral multiple plus one-half waveguide. wavelengths long at a second microwave frequency. The signal at the first frequency is coupled into the waveguide loop and circulates around the loop, is coupled from the waveguide loop and propagates from an output terminal on the second waveguide. The signal at the second microwave frequency propagates through the first. waveguide and appears at an output terminal on the first waveguide. By employing a directional cou pler having high directivity and a low coupling coefficient to cause a traveling wave to circulate around the wave guide loop, and by employing a similar directional coupler to extract energy from the waveguide loop, high selectivity is ach eved at the output terminal of the separated frequency, as will be more fully explained hereinafter.

For a better understanding of the invention reference should be made to the acornpanying drawings wherein:

FIG. 1 is a diagrammatic representation of the frequency separator of the present invention;

FIGS. 2, 3 and 4 are graphical curves used in explaintnt i9? sen-sis Patented Jan. 15, 1953 ing the theory of operation of the frequency separator; and

FIG. 5 is a diagrammatic representation of an embodiment of this invention employed to separate a plurality of signals at different microwave frequencies.

Referring now more particularly to FIG. 1 there is shown a section of rectangular waveguide 10 in the form of a closed loop having two rectangular waveguide sec tions 11 and 12 coupled to waveguide loop 10 by means of directional coupling slots 13 and 14, respectively. As shown in FIG. 1 waveguide 12 is located at a region on Waveguide loop lit opposite waveguide 11. However, as will become apparent from the description which follows, this arrangement is merely illustrative of one embodiment of the invention and the successful operation of the frequency separator of this invention is not dependent upon waveguide 12 being located opposite waveguide 11. An input terminal 20 is connected to one end of waveguide 11, and an output terminal 22 is connected to the opposite end. Waveguide 12 has an output terminal 21 connected at one end and the opposite end is terminated in an energy absorbing load 23. A microwave energy source 2-1 is connected to the frequency separator at terminal 2%. 1

A phase adjusting means 15 may be placed in Waveguide loop ltl, as will be more fullyexplained hereinafter.

it is to be understood that directional coupling slots 13 and 14 in connection with waveguide ring it} and Waveguides 11 and 12 are merely illustrative of one means of providing directional coupling between waveguides 11 and i2 and waveguide loop 10. Other directional coupling means may be employed equally as well without departing from the scope of the present invention.

in addition, the frequency separator of this invention will operate if coupling means other than a directional coupler is employed in the place of directional coupling slots 14. However, the use of a directional coupler illustrates the preferred embodiment for the practice of this invention in that coupling means other than a directional coupler is more likely to create a mismatch, thus creating undesirable standing waves in loop 10.

The directional couplers employed in the practice of this invention should have high directivity to ensure optimum performance of the frequency separator of this invention. Directivity is a term used to describe a characteristic of a directional coupler and may be defined by D:l0 log P /P where P is the power coupled in the preferred direction in the secondary line of a directional coupler and P is the power coupled in the non-preferred direction in the secondary line of a directional coupler. Directivities of 20 db or greater will permit successful operation of the frequency separator of this invention.

In the operation of the frequency separator of this invention consider that microwave energy in the TE mode, having signals at two different frequencies, F and F is incident at terminal 29, and that phase adjusting means 15 is adjusted so that waveguide loop 10 is an integral number of waveguide wavelengths long (mt at frequency F and an integral number plus one-half waveguide Wavelengths long (n+ /2 at frequency F The Wave of energy at frequency F will enter terminal ill and will propagate to the right in waveguide 11 where it will encounter directional coupling slots 13 and will be divided into two components, a direct component and a coupled component. The direct component will continue along waveguide 11 toward output terminal 22, while the coupled component will be coupled into waveguide loop 10 by the coupling slots 13. Because of the highly directional characteristic of coupling slots 13 substantially all of the coupled component of the energy at frequency F will propagate in a clockwise direction in waveguide loop 10, and a negligible amount of the energy will propaga'te in a counterclockwise direction. Considering only the phase shift .of the two components relative to each other, the coupled component will experience a 90 phase shift in relation to the direct component in passing through coupling slots 13.

The coupled component of energy at frequency F propagating around waveguide loop will next encounter directional coupling slots 14 and will be divided into direct and coupled components, the direct component traveling in waveguide loop 10 past the coupling slots, and the coupled component being coupled into waveguide 12 by the directional coupling slots 14. Because of the directivity of coupling slots 14 the coupled component will propagate to output terminal 21. Energy absorbing load 23 will absorb any energy which tends to couple in the opposite direction in waveguide 12.

The energy coupled from output terminal 21 will have experienced a total phase shift of rt/ 360+ 180, where n is any integer; 90 phase shift in passing through coupiing slots 13, 90 phase shift in passing through coupling slots 14, and lit/ x360 phase shift in traveling around one-half of waveguide loop 10.

The direct wave at coupling slots 14 will pass by the coupler with substantially zero additional phase shift and will continue to propagate around the bottom of wave guide loop 10 until it encounters coupling slots 13 where it will be divided into a direct wave and a coupled wave. The direct wave will continue by the coupler without additional phase shift and will commence to recirculate around waveguide loop 10. Because the electrical length of loop 10 is an integral number of waveguide wavelengths long at frequency F the wave which commences to recirculate around waveguide ring 10 will have experienced a total phase displacement of 90, having been coupled through coupling slots 13 once. This circulating wave will combine with the wave which has traveled down waveguide section 11 and is now being coupled into loop 10 by coupling slots 13. Both of these waves will have a phase displacement of'90, each having passed once through coupling slots 13, so that they will combine in phase and will continue to circulate around waveguide loop 10 as a traveling wave. Now going back to the wave which was traveling around the bottom of loop 10 for the first time and considering its coupled component when it encounters coupling slots 13, this coupled component will enter waveguide 11 and because of the directivity of coupling slots 13 will propagate to the right to- Ward terminal 22. This coupled component will experience an additional 90 phase shift in going through coupling slots 13, and will have a total phase displacement of 180, since it has passed through coupling slots 13 twice.

This coupled wave at 180 will now combine in waveguide 11 with the direct wave of energy from input terminal 20 which is passing coupling slots 13 for the first time, and because this direct wave experiences substantially zero phase shift in passing by coupling slots 13, the two waves will be 180 out of phase and will partially cancel.

After many circulations of the traveling wave around waveguide looplt) there will be a power build-up in loop 10, similar to the power build-up described in copending application Serial No. 482,076, filed January 17, 1955, by Peter J. Sferrazza, now Patent 2,875,415, and assigned to applicants assignee. For an explanation of this power build-up in waveguide loop 10, reference is made to the Sferrazza patent.

As a result of the power build-up due to many circulations of the coupled waves in loop 10, the wave coupled from loop 10 into waveguide 11 by coupling slots 13 substantially cancels the direct wave in guide 11 passing directly by coupling slots 13 so that very little energy at frequency P; will be present at output terminal 22. How- 'ever, a portion of the large field strength at frequency F which'is built up in loop '10 will couple through coupling slots 14 and will propagate from output terminal 21. Therefore, microwave energy at frequency F will propagate from output terminal 21 and very little will propagate from output terminal 22.

Now consider the operation of the frequency separator of this invention at frequency F remembering that the electrical length of waveguide loop 10 is an integral number plus one-half waveguide wavelengths long at frequency F A wave of microwave energy at frequency F will enter input terminal 20 and will propagate to the right in waveguide 11 and will encounter coupling slots 13 where it will be divided into a coupled component which is coupled into waveguide loop 10 and into a direct component which propagates in waveguide 11 directly by the coupling slots 13.

The coupled component of the wave at frequency F will circulate around waveguide loop 10 as did the wave at frequency F except that when the wave in loop 10 encounters coupling slots 13 after the first circulation, the wave will now have a total phase displacement of 270, having been introduced by the coupling action at slots 13, and having been introduced because of the electrical length of waveguide loop 10.

When the circulating wave at frequency F encounters coupling slots 13 it will divide into a direct component and a coupled component. The direct component will pass by the slots 13 and will continue in loop 10, and the coupled component will couple into waveguide 11 and will experience a phase shift of 90 in going through coupling slots 13. The component coupled into waveguide 11 now has a total phase displacement of 360 and will combine with the direct wave from input terminal 20 which passes by the coupling slots 13. These two waves are in phase with each other and will conbine to propagate from output terminal 22.

The direct wave of circulating energy in loop 10 having a phase displacement of 270 will combine with energy from input terminal 20 which is coupled from waveguide 11 into waveguide loop 10 and since the new energy is displaced in phase by 90 in passing through coupling slots 13, the two waves are 180 out of phase and will partially cancel. Again because of the power build-up in loop 10 the two components of energy at frequency F will substantially cancel after many circulations around loop 10 and very little energy at frequency F will couple from loop 10 to the output terminal 21.

Thus combining the response of this frequency separator to waves at frequencies F and F it is seen that energy at frequency F will propagate from output terminal 21 and very little will propagate from output terminal 22, while energy at frequency F will propagate from output terminal 22 and very little will propagate from output terminal 21.

From the above explanation of the operation of this frequency separator an advantageous feature of this device becomes evident. This feature is that the impedances at all terminals of the device are relatively constant over a broad band of frequencies and do not exhibit the rapid change with frequency that is exhibited by some conventional filters. This behavior results from the fact that the input energy at terminal 20 is separated into its frequency components and these components are coupled to output terminals 21 and 22, whereas in many of the more conventional filters frequency selectivity is obtained by reflecting back into the input terminals the non-resonant frequencies incident upon the filters.

As was pointed out earlier in this description the frequency separator of this invention is highly selective. This highly desirable feature is a consequence of employing directional couplers, and from the fact that a traveling wave is set up in waveguide loop 10.

The high selectivity of this frequency separator will now be explained more fully.

For purposes of analysis, assume that the directional couplers 13 and 14 have infinite directivity. At plane A-A, FIG. 1, the circulating wave traveling in waveguide loop 10 (E is expressed as Ea: [G1E1-i-C'LE1e' (l-C1 (1O2 +O'1E1e (lC1 )(1O-2 where C is a constant and represents the coupling provided by coupling slots 13, C is a constant and represents the coupling provided by coupling slots 14, 'y=u+jfi, where on represents the attenuation of a signal in one circulation around waveguide loop 10, 5 represents the phase shift of a signal in one circulation around waveguide loop 10, and E represents a wave of energy at a given frequency, say F incident at terminal 20.

The first term of the expression (C E is the component of energy coupled from waveguide 11 into loop 10 by coupling slots 13. The second term of the expression is the component of energy in loop it after one circulation around waveguide loop 10. The third term of the expression 'From FIG. 1 it is seen that the component of energy coupled from waveguide loop 19 to output terminal 21 (E may be expressed as After combining Equation 3 with Equation 2, the ratio of the output wave at terminal 21 to the input wave at terminal 20, the coupling from terminal 2% to terminal 21 (C may be expressed as E C EC [l-(1C' (1C e" If the device of this invention is completely symmetrical, C is equal to C and Equation 4 will reduce to A plot of Equation 5 is shown in FIG. 2 for several different values of coupling.

In this figure the abscissa is expressed in the electrical length of waveguide loop 1%. The value of a was assumed to be .1 db. In the above equatio-nsa is expressed in nepers.

It is evident from the curves of FIG. 2 that the selectivity at terminal 21 of this device increases as the coupling is decreased, and that the selectivity is quite high when C is equal to .01 db. This property is a salient feature of this invention and is a feature which distinguishes it from prior art microwave filters of comparable size and complexity. In known devices employing hybrid junctions the values of coupling are greater than can be achieved with directional couplers, thus resulting in poorer selectivity.

The response at terminal 22 will be the inverse of that shown in FIG. 2 and is illustrated in FIGS. 3 and 4.

The curves of FIGS. 2, 3, and 4 illustrate that this frequency separator is highly selective at terminal 21, and that the characteristics at terminal 22 are that of a broad band-pass filter having high skirt selectivity. This feature makes this device ideally suited to separate microwave energy having more than two different frequencies.

For example, in FIG. 5, microwave energy having signals at frequencies F F F and F enters transmission line 100 and propagate to the right. Transmission line loop 101 is an integral number of electrical wavelengths long at frequency F transmission line loop 102 is an integral number of electrical wavelengths long at frequency F and transmission line loop N3 is an integral number of electrical Wavelengths long at frequency F Each loop of the structure shown in FIG. 5 will function in the same manner as the loop of FIG. 1 operates, so that loop 101 will separate frequency F and this frequency will propagate from output terminal 104 while energy at frequencies F F and F will continue to propagate to the right in transmission line 100. As the energy continues to the right in transmission line 100, frequency F will be separated by loop 102 and will propagate from terminal 105, and frequency F will be separated by loop 103 and will propagate from terminal 106. The remaining signal at frequency R; will propagate from terminal 109. The directional couplers employed to couple the separated frequencies may be forward firing couplers as illustrated in FIG. 5, or may be backward firing couplers, depending upon the requirements of the designer.

The response characteristics of the device of this in vention also indicate that the device may be employed as a band-pass filter or as a band-stop filter. As is evident from the curves of FIGS. 3 and 4, the response at terminal 21 is that of a narrow band-pass filter and the response at terminal 22 is that of a narrow band-stop filter.

Any type of energy transmission line such, for example, as coaxial transmission lines, stripline transmission lines and the like may be used in the place of the waveguide transmission line illustrated herein. Therefore, the use of the'word waveguide in the appended claims is intended to include all transmission lines capable of guiding electromagnetic waves.

While the invention has been described in its preferred embodiment, it is to be understood that the words which have been used are words of description rather that of limitation and that changes within the purview of the appended claims may be made without departing from the true scope and spirit of the invention in its broader aspects.

What is claimed is:

l. A microwave frequency separator for separating two waves of different predetermined frequencies supplied from a microwave energy source, said separator comprising a closed loop wave transmission line, said transmission line being substantially an integral number of wavelengths long at a first one of said frequencies, a second wave transmission line adapted to be connected at one end to the source and at the opposite end to a first output terminal, a directional coupling means for coupling said closed loop to said second transmission line at a region intermediate the ends of said second transmission line, a third wave transmission line having one end connected to a second output terminal, and coupling means for coupling said closed loop to said third transmission line at a region intermediate the ends of said third transmission line, at least one of said coupling means for coupling said closed loop to a respective one of said second or third transmission lines having a degree of coupling less than one-half.

2. A frequency wave-filter comprising a first waveguide, input means to provide electric waves in said waveguide, output means comprising a load coupled to said waveguide, a waveguide system providing a loop-shaped path substantially closed upon itself for electric waves, a directional coupler connecting said first waveguide with said waveguide system and having a degree of coupling less than one-half so as to couple a relatively small portion of the electric waves in said first waveguide into said Waveguide system and to couple a relatively small portion of the electric waves in said waveguide system into said first waveguide, and means for partially attenuating the electric waves in said waveguide system, said last-named means 7 comprising a second waveguide, a second directional coupier connecting said second waveguide to said Waveguide system and having a degree of coupling less than one-half so as to couple a relatively small portion of the electric waves in said Waveguide system into said second waveguide, and a load coupled to said second waveguide.

References Cited in 'the file of this patent UNITED STATES PATENTS Sunstein Feb. 15, 1955 Van de Lindt Dec. 20, 1955 Zaleski July 31, 1956 Dicke Sept. 11, 1956 Kock Aug. 26, 1958 

1. A MICROWAVE FREQUENCY SEPARATOR FOR SEPARATING TWO WAVES OF DIFFERENT PREDETERMINED FREQUENCIES SUPPLIED FROM A MICROWAVE ENERGY SOURCE, SAID SEPARATOR COMPRISING A CLOSED LOOP WAVE TRANSMISSION LINE, SAID TRANSMISSION LINE BEING SUBSTANTIALLY AN INTEGRAL NUMBER OF WAVELENGTHS LONG AT A FIRST ONE OF SAID FREQUENCIES, A SECOND WAVE TRANSMISSION LINE ADAPTED TO BE CONNECTED AT ONE END TO THE SOURCE AND AT THE OPPOSITE END TO A FIRST OUTPUT TERMINAL, A DIRECTIONAL COUPLING MEANS FOR COUPLING SAID CLOSED LOOP TO SAID SECOND TRANSMISSION LINE AT A REGION INTERMEDIATE THE ENDS OF SAID SECOND TRANSMISSION LINE, A THIRD WAVE TRANSMISSION LINE HAVING ONE END CONNECTED TO A SECOND OUTPUT TERMINAL, AND COUPLING MEANS FOR COUPLING SAID CLOSED LOOP TO SAID THIRD TRANSMISSION LINE AT A REGION INTERMEDIATE THE ENDS OF SAID THIRD TRANSMISSION LINE, AT LEAST ONE OF SAID COUPLING MEANS FOR COUPLING SAID CLOSED LOOP TO A RESPECTIVE ONE OF SAID SECOND OR THIRD TRANSMISSION LINES HAVING A DEGREE OF COUPLING LESS THAN ONE-HALF. 